1. Field of the Invention
This application relates generally to electrically powered and controlled tools for moving and operating equipment within passages, such as cased wells and open boreholes.
2. Description of the Related Art
It is known to deploy various types of tools for moving and operating equipment in passages, such as wells and open boreholes. In oil and gas wells, such equipment is often referred to as a “bottom hole assembly” and can perform various functions, which may or may not require fluid for operation. Functions that typically require fluids include drilling, acidizing, and sand washing, and functions that typically do not require fluids include logging of open and cased boreholes, conducting pressure and temperature surveys, and caliper logs.
As used herein, the terms “hole,” “passage,” “well,” and “borehole” are used interchangeably. The inner perimeter of a hole is referred to herein as a “surface,” “inner surface,” or “wall” of the hole. A cased hole is one that has a casing or metal liner (such as so-called sand screen) formed at its inner surface. An open hole is one that does not have such a casing. As used herein, the term “downhole” refers to the direction pointing away from a ground surface at which a tractor is deployed, and the term “uphole” refers to the direction pointing toward the ground surface.
A tractor is one type of tool that can move and help to operate equipment in passages. A tractor may include an elongated body, one or more gripper assemblies (also sometimes referred to as “grippers”) along the body, and one or more propulsion assemblies. Each gripper assembly may have a radially expanded position in which the gripper assembly limits relative movement between the gripper assembly and an inner surface of a passage, well, or borehole. Each gripper assembly can also have a radially retracted position in which the gripper assembly permits substantially free relative movement between the gripper assembly and the inner surface of the passage. Each propulsion assembly can produce longitudinal displacement of the body with respect to one of the gripper assemblies when radially expanded.
In certain implementations, tractors are adapted to walk through a borehole or well. Typically, a first gripper assembly is expanded to grip the hole, and a propulsion assembly propels the tractor body longitudinally with respect to the expanded first gripper assembly. This is referred to as a “power stroke” with respect to the first gripper assembly. Simultaneously, a retracted second gripper assembly is moved longitudinally with respect to the body for a subsequent power stroke. This is referred to as a “reset stroke” with respect to the second gripper assembly. After these power and reset strokes complete, the second gripper assembly is expanded and the first gripper assembly retracts. Then, a propulsion assembly propels the tractor body longitudinally with respect to the expanded second gripper assembly. In other words, the tractor conducts a power stroke with respect to the second gripper assembly. Simultaneously, the retracted first gripper assembly is moved longitudinally with respect to the body for a subsequent power stroke. In other words, the tractor conducts a reset stroke with respect to the first gripper assembly. Tractors that employ this walking method include those described in U.S. Pat. Nos. 6,003,606 to Moore et al.; U.S. Pat. No. 6,241,031 to Beaufort et al.; U.S. Pat. No. 6,347,674 to Bloom et al.; U.S. Pat. No. 6,679,341 to Bloom et al.; U.S. Pat. No. 7,121,364 to Mock et al.
Many known tools use fluid to expand the gripper assemblies and to propel the tool longitudinally within a borehole or well. In so-called open systems, the fluid is typically pumped from the ground surface to the tool through coiled tubing or jointed pipe that is connected to an aft end of the tool. Such fluid typically exits the tool into the annulus between the tool and the hole wall, and then returns to the ground surface through the borehole or well. In closed systems, the fluid is contained within the tool and simply circulates therein. Fluid-powered tools are particularly useful when the tool's payload (i.e., the equipment that the tool moves through the hole) is heavy, such as perforation guns for forming holes within a well casing. Fluid-powered tools are also useful when the hole that is being serviced is extremely long (e.g., 20,000-35,000 feet).
Other known tools are powered entirely electrically. Such tools are employed within wells, as opposed to open (i.e., uncased) boreholes. Such tools can employ wheels or moving traction belts for gripping and moving with respect to the inner surface of a cased well. Such tools often employ downhole electric motors that perform operations related to moving the tool downhole. Electrical power and signals for propelling and controlling the tool is normally provided through a wireline that extends from the ground surface to the tool, through the well. Electrically powered tools (or “wireline tools”) are preferred when payloads are relatively light (e.g., less than 2000 lbs) and the hole to be serviced is not extremely long. Examples of lighter payloads include logging tools and certain pipeline applications.
Tractors push and/or pull a bottom hole assembly through a passage. A tractor utilizing a wireline, coiled tubing, or jointed pipe must also be able to pull it through the passage, including overcoming frictional drag forces thereon.
Certain types of downhole equipment are powered only electrically and controlled only electronically. This equipment is generally more compatible with downhole tools and tractors that are likewise powered only electrically and controlled only electronically. Thus, for many applications, fluid-powered tractors may be less preferred for these compatibility reasons.